Selective catalytic reduction reactors are used to convert nitrous oxides (NOx), to nitrogen (N2) and water (H2O). The selective catalytic reduction process is typically carried out to prevent the nitrous oxides from entering and polluting the atmosphere. Nitrous oxides are produced by different types of industrial equipment such as boilers, engines, and turbines. For example, in power stations that use coal to generate electricity, flue gases emanating from boilers in the power stations contain nitrous oxides.
The selective catalytic reduction process involves adding a reductant to the flue gas and passing the flue gas through a bed of catalyst to convert the nitrous oxides. The reductant may include ammonia or urea and the catalysts may include zeolites, metal oxides such as vanadium oxide and titanium oxides, and the like.
For boilers powered by coal, ash produced as a result of burning the coal may be transported by the flue gas to the catalyst, where the ash may bind to and plug the catalyst. Ash typically comprises silicon dioxide, calcium oxide, carbon and many other constituents depending on the makeup of the coal being burned. The combustion ash particles are usually small (up to 300 micro meters in diameter) and so they are easily suspended in the flue gas. However, the combustion ash particles can form large particle ash (LPA), which may have a diameter exceeding 1 centimeter. The LPA can plug the openings in the catalyst. Thus, screens are provided to remove LPA particles from the flue gas before they get to the catalyst. For example, patent application Ser. No. 13/633,717, entitled, “Apparatus and Methods for Large Particle Ash Separation From Flue Gas Using Screens Having Semi-Elliptical Cylinder Surfaces,” filed Oct. 2, 2012 to Buzanowski et al., describes screens for separating ash particles from the flue gas. In addition to such screens for separating LPA from the flue gas, selective catalytic reduction reactors are provided with protective catalyst wire mesh screens that are placed above the catalyst. The catalyst is a very expensive component of the selective catalytic reactor and the catalyst wire mesh screen is designed to allow personnel, when the reactor is not in operation, to walk in the reactor, on top of the catalyst wire mesh screen, without damaging the catalyst. In some instances, the catalyst wire mesh screen is designed to have slightly smaller openings than the openings in the catalyst so that there is a buildup of ash particles from the flue gas on the catalyst wire mesh screen instead of the catalyst.
FIG. 1 shows prior art selective catalytic reduction reactor system 10. Selective catalytic reduction reactor system 10 shows equipment related to a selective catalytic reduction process in a typical coal fired power plant. In boiler 100, coal is mixed with air (from preheater 101) and burned. The burning coal causes an increase in temperature in boiler 100 so that water injected into boiler 100 is vaporized to form steam. The burning coal produces ash particles 102 (including LPA 102A and fly ash 102B), which flows with hot flue gas 103 through duct 104A. Duct 104A leads to LPA screen separator 105. LPA screen separator 105 has holes having a diameter so that flue gas 103 and fly ash 102B pass through LPA screen separator 105. However, LPA 102A particles are too big to pass through the holes of LPA screen separator 105.
Because these LPA 102A particles are too big to pass through the holes of LPA screen separator 105, they accumulate in hopper 106. Flue gas 103 and fly ash 102B passes through LPA screen separator 105 and enters duct 104B. Duct 104B channels flue gas 103, fly ash 102B, and reductant 107 to selective catalytic reduction (SCR) reactor 108. SCR reactor 108 removes nitrous oxides from flue gas 103 by converting the nitrous oxides to nitrogen and water in a reduction reaction. Catalyst bed 109 facilitates this conversion by speeding up the reduction reaction when flue gas 103, fly ash 102B, and reductant 107 are passed through catalyst bed 109. Flue gas 112, leaving catalyst bed 109, has a reduced amount of NOx compared with flue gas 103 and is discharged into the atmosphere or cleaned further and then discharged into the atmosphere.
Ash particles small enough to pass through LPA screen separator 105 (e.g., fly ash 102B) may accumulate on catalyst wire mesh screen 110 or in SCR catalyst bed 109. The accumulation of fly ash 102B on catalyst wire mesh screen 110 and/or in SCR catalyst bed 109 may negatively affect the performance of SCR catalyst bed 109 and the catalytic reduction process. Thus, when fly ash 102B accumulates sufficiently on catalyst wire mesh screen 110, cleaning equipment 111 may be used to clear away those ash particles. Cleaning equipment 111 may include air cannon cleaning equipment and/or sonic horn cleaning equipment.
The sonic horn is a low frequency, high energy acoustic horn used as a cleaning mechanism. When the sonic horn emits low frequency sound, the sound waves vibrate fly ash (e.g., fly ash 102B) and dislodge it from where it has settled. If the sound doesn't dislodge fly ash 102B, it just vibrates and gets packed into the catalyst wire mesh screen. Because of the rough finish of catalyst wire mesh screen 110, the sonic horns tend to displace and lodge fly ash 102B into catalyst wire mesh screen 110 and create a buildup of the fly ash. If the sonic horns sufficiently vibrate and dislodge fly ash 102B particles, then either gas flow or gravity moves the fly ash 102B deposits away from catalyst wire mesh screen 110. But when the sonic horns are not able to vibrate fly ash 102B particles enough to get them off catalyst wire mesh screen 110, then fly ash 102B particles get packed in and on catalyst wire mesh screen 110.
Generally, fly ash 102B particles are smaller than the holes in catalyst wire mesh screen 110. Catalyst wire mesh screen 110 is designed so that most of fly ash 102B particles flow through the mesh wire and through the catalyst. However, because of the rough surfaces on catalyst wire mesh screen 110, fly ash 102B particles get attached to the rough surfaces and then fly ash 102B particles are packed on catalyst wire mesh screen 110 by the sonic horns.
The performance of SCR catalyst bed 109 may also be affected by unequal flue gas velocity flowing through SCR catalyst bed 109. Such unequal flue gas velocity flow may erode SCR catalyst bed 109. High velocity flue gas flow through the catalyst erodes and damages the catalyst. On the other hand, if the flue gas velocity is too low, it may cause fly ash 102B particles to accumulate on catalyst wire mesh screen 110. This may cause a buildup of piles of fly ash particles above catalyst bed 109 on catalyst wire mesh screen 110. Such fly ash piles can cause angled flow of the flue gas and can increase velocity of the flue gas, both of which may cause or exacerbate erosion of the catalyst in SCR catalyst bed 109. Further, uneven velocity distribution may affect residency and effectiveness of catalyst to fully catalyze chemical reaction.